The basic objective of this research is to improve the radiotherapy of brain tumors. Using alkaline sucrose gradients in zonal rotors, we have recently demonstrated that intracerebral rat 9L tumor cells restore their chromosomal DNA structure more rapidly after x-irradiation than do the normal nondividing cerebellar neurons. Therefore, at any time after radiotherapy the DNA in tumor cells can be expected to be repaired to a greater extent than the DNA in the normal nondividing cells. If radiotherapy schedules are to be designed so that the accumulation of DNA damage in normal tissue is minimized, then critical periods in repair process must be found such that a second dose of radiation or the addition of a drug at the critical repair period will cause more damage in the tumor than in the normal brain. We therefore are using the zonal rotor technique: 1) to determine the in situ DNA damage and repair kinetics of the intercerebral 9L brain tumor and cerebellar neurons in Fischer 344 rats after split doses of ionizing radiation, 2) to seek ways of modifying the DNA repair kinetics of tumor and normal cells by proper timing of radiation treatments or by the administration of various cell cycle nonspecific (i.e., BCNU, CCNU) or cell cycle specific (5-FU, hydroxyurea) drugs, and 3) to attempt to correlate the accumulation of structural DNA damage with the loss of cellular clonogenic or functional capacity.